1. Field of the Invention
The present invention relates to a thin film head to be used as a head of a magnetic recording/reproducing apparatus.
2. Description of the Related Art
Recently, as the density of magnetic recording has increased more and more, systems with a high recording density of 500 Mb/inch.sup.2, as a VTR, and 200 Mb/inch.sup.2, as an HDD, have been put into practical use. These systems primarily make use of an induction type magnetic head. In recent years, however, a thin film head is beginning to be used in place of the induction type magnetic head not only in systems for reproducing fixed head type tape media but in compact HDDs with a low relative velocity of several meters/sec, since the magnetoresistance effect head has a high S/N ratio.
Recently, it is known that a very large magnetic resistance change, a magnetic resistance change of a maximum of larger than 100%, appears in a multilayered film, the first example is an artificial lattice film which is formed by stacking ferromagnetic films and nonmagnetic films, such as Fe/Cr or Co/Cu, under certain conditions (Phys. Rev. Lett., Vol. 61, 2472 (1988), Phys. Rev. Lett., Vol. 64, 2304 (1990)). It is also reported that the rate of change in magnetic resistance varies periodically when the thickness of the nonmagnetic film is changed. It is explained that this change is brought about since the ferromagnetic film periodically experiences ferromagnetic coupling or antiferromagnetic coupling depending on the thickness of the nonmagnetic film. The electrical resistance of the stacked film is high in an antiferromagnetic coupled state and low in a ferromagnetic coupled state.
The second example is a system in which two types of films with different coercive forces are used, and a magnetic resistance change is realized by setting magnetizations of these two magnetic films in an antiparallel state by using the difference in coercive force (The Journal of The Japan Applied Magnetics Society, Vol. 15, No. 5, 813 (1991), a so-called Shinjo type).
The third example is a system in which an exchange bias generated by an antiferromagnetic film is applied to one of two magnetic films sandwiching a nonmagnetic film to thereby lock magnetization of that magnetic film, and magnetization of the other magnetic film is reversed by an external magnetic field. This realizes a large change in a magnetic resistance by producing states in which the magnetizations of the magnetic films are parallel and antiparallel to each other via the non-magnetic film (Phys. Rev. B, Vol. 45806 (1992), J. Appl. Phys., Vol. 69, 4774 (1991), a so-called spin valve type).
FIG. 1 shows a conventional magnetoresistance effect element.
This conventional magnetoresistance effect element has a contact region B for connecting a lead 101 on each side of a region A corresponding to a track width Since a high-permeability soft magnetic film 102d which responds to magnetization of a medium exists in this region B, this portion also senses recorded information. For this reason, information from the adjacent track is also mixed upon off-track, and this reduces the S/N ratio and makes the track width obscure. When the recording density is 200 Mb/inch.sup.2, for example, the track width is 7 .mu.m, and the track spacing is about 2 .mu.m. In this case, since the track width is relatively large and the space between track is also large, the contact region B does not exist on the neighboring track if the width of the contact region B is set to about 1 .mu.m or less. Therefore, a leakage output (crosstalk) from the neighboring track is negligible in off-track with a track spacing of 1 .mu.m or less.
In FIG. 1, numeral 102a represents an undercoat film, numeral 102b represents a soft magnetic film for applying a bias, numeral 102c represents a nonmagnetic film, and numeral 102e represents a protective film.
If, however, the recording density is, e.g., 10 Gb/inch.sup.2, the track width and the track spacing decrease to approximately 1 and 0.2 .mu.m, respectively, and so the output itself also decreases. This makes the contact region B to exist on the adjacent track upon off-track, and the leakage output from the adjacent track can no longer be neglected. To avoid this inconvenience, the width of the contact region B may be decreased to about 0.2 .mu.m which is equal to the track spacing. In this case, however, imperfect ohmic contacts readily form in mass production.
The first object of the present invention is as follows.
As described above, as the recording density approaches 10 Gb/inch.sup.2, the output leaking from the neighboring track through the contact region B upon off-track becomes no longer negligible. If the area of the contact region B is decreased in order to avoid this leakage output, there arises another problem of the difficulty in forming even ohmic contacts.
In the magnetoresistance effect element, use of a magnetoresistance effect film with a high antiferromagnetic coupled state increases the saturation magnetic field because of a high coupling force of the film. Therefore, there have been reported several systems which use a phenomenon, in which the resistance changes between a parallel magnetization state and an antiparallel magnetization state, rather than the antiferromagnetic coupled state.
The second object of the present invention is as follows.
Another method of precisely defining the track width in a magnetic head is to use a conventional magnetoresistance effect element which extends in the direction of applying the signal magnetic filed as is shown in FIG. 2A or in the opposite direction as shown in FIG. 2A, to acquire an improved sensitivity. Whichever direction the magnetoresistance effect element extends, however, no uniform bias magnetic field can be applied to the magnetoresistance effect element. This is because the magnetoresistance effect element has its resistance-changed with the angle between its axis of magnetization and the curved path of sense current. Consequently, the magnetoresistance effect element cannot provide a reliable output. It cannot be used in practice, particularly for tracks which are so narrow that the sense-current path is curved greatly.
Conventional magnetoresistance effect elements include a type constituted by two magnetic films sandwiching a nonmagnetic film as shown in FIGS. 2B and 2C (J. Appl. Phys. 53(3), 2596, 1982). Referring to FIGS. 2B and 2C, reference numeral 105 denotes a lower magnetic film; 106, a nonmagnetic film; 107, an upper magnetic film; and 108a and 108b, leads. In FIGS. 2A and 2B, a sense current flows into the lead 108a and out from the lead 108b. The magnetoresistance effect element with such an arrangement is affected by a magnetic field generated by a self-current. Assuming that a magnetic field generated by a current flowing through the lower magnetic film and the nonmagnetic film is applied to the upper magnetic film, the magnitude of the magnetic field generated by the self-current when the film thickness of each layer is equal to or smaller than the mean free path of conduction electrons is given by Relation (1) below. Note that the mean free path is approximately 300 .ANG. for 300 K in the case of bulk Cu. EQU H.sub.x1 to J*(t+d)/2 . . . (1)
wherein J represents a current density, t represents a thickness of the upper magnetic film, and d represents a width of the upper magnetic film.
As an example, H.sub.x to 700 (A/m)=9 (Oe) when J=2.times.10.sup.7 A/cm.sup.2, t=20 .ANG., and d=50 .ANG..
A magnetic field H.sub.x2 applied to the lower magnetic film is given by H.sub.x2 =-H.sub.x1 because it is generated by a current flowing through the upper magnetic film and the nonmagnetic film.
Under these conditions, therefore, the magnetic moments (magnetizations) of the upper and lower magnetic films are antiparallel to each other if an anisotropic magnetic field of each layer is small (up to 3 Oe) like that of a thin permalloy film. However, a domain wall called an edge curling wall is present in an edge portion of the element due to the influence of a demagnetizing field (IEEE Trans. Magn., Vol. 24, No. 3, May 1988), so this portion essentially becomes a dead region.
That is, the performance of the magnetoresistance effect element as described above can be maximally exhibited if the magnetic moment of each individual layer is uniform in the layer. However, if the edge curling wall exists, the magnetic moment cannot be uniform any longer, and the response of this portion to an external magnetic field is degraded. Consequently, the reproduced output from this magnetoresistance effect element decreases by a ratio corresponding to the width of the edge curling wall.
When magnetic anisotropy is imparted in the width-wise direction of the element, the width of this edge curling wall is obtained by Equation (2) below (IEEE Trans. Magn. Vol. 24, No. 3, May 1988): EQU .pi..DELTA./2={.pi..sup.3 M.sub.s *d*t/(2H.sub.k)}.sup.0.5 . . . (2)
where M.sub.s is the saturation magnetization and H.sub.k is the magnitude of an anisotropic magnetic field. For example, if H.sub.k =3 Oe and M.sub.s =800 G in the above element, .pi..DELTA./2=0.2 .mu.m. When a sense current is flowed, H.sub.k in Equation (2) becomes H.sub.k +H.sub.x, and consequently .pi..DELTA./2=0.1 .mu.m.
In ultra-high-density magnetic recording with a surface density of 10 Gb/inch.sup.2, the area per bit is about 0.07 .mu.m.sup.2, and the track width is 1 .mu.m or less. In this case, the dimensions of the element also decrease to a similar extent (a square of 1 .mu.m side). Therefore, a ratio that the width of the edge curling wall occupies 20% of the whole element; that is, only an 80% portion of the element is usable as an active region. This problem similarly arises in an element in which magnetic films and nonmagnetic films are stacked alternately as well as in an element having two magnetic films.
The third object of the present invention is as follows.
As discussed above, in the magnetoresistance effect element formed by alternately stacking magnetic layers and nonmagnetic films, the magnetic moment of each individual layer in an edge portion of the element tends to be parallel to this edge portion, producing an edge curling wall. When the element is miniaturized, a ratio that this edge curling wall occupies in the entire element increases. In an extreme case, magnetization near a central portion also acquires a component parallel to the edge portion. This lowers the reproduced output from the magnetoresistance effect element.
If an antiferromagnetic film is arranged on at least one side of the above magnetoresistance effect element to apply an uniaxial biasing in the widthwise direction of the element, this edge curling wall can be eliminated. Since, however, an FeMn-based antiferromagnetic film currently being developed in practice has a very low corrosion resistance, it is not possible to make an arrangement in which this antiferromagnetic film is exposed to the atmosphere.
Conventional thin film heads using a permalloy or the like have a practical problem of Barkhausen noise which is derived from an irreversible behavior of a magnetic domain appearing in a magnetic film. As a technique to prevent this problem, there has been proposed a method by which an exchange bias is obtained by stacking antiferromagnetic films consisting of, e.g., FeMn, or a method by which a magnetic substance is allowed to have a single magnetic domain by arranging a magnetic material in the vicinities of both the edges of an element (IEEE MAG-14, 521 (1978), Jpn. Pat. Appln. KOKAI Publication No. 64-1112).
The fourth object of the present invention is as follows.
In the above conventional technique, a single-layered magnetic film or a multilayered magnetic film in which magnetizations of individual magnetic layers point in the same direction can be allowed to have a single magnetic domain by applying a bias magnetic field in one direction. If, however, a magnetic film is constituted by three or more layers and magnetizations of these magnetic layers do not point in the same direction, there is no means for forming a single magnetic domain to each magnetic layer.
The fifth object of the present invention is as follows.
A thin film head is known, wherein the direction of the signal magnetic field is substantially parallel to the direction of the sense current supplied to detect resistance changes, leads are arranged on the surface which opposes the recording medium and at which the magnetic flux density is maximal. To improve the magnetic permeability of the magnetoresistance effect element, the element is elongated about 10 .mu.m in the direction in which the sense current flows, thereby reducing the intensity of the anti-magnetic field extending in the direction of the signal magnetic field. Notwithstanding, in the case of a so-called shield-type magnetoresistance effect head, the magnetic flux form the recording medium leaks into the shield at 1 to 2 .mu.m from the surface of the recording medium. Thus, the resistance changes in a region extending only 1 to 2 .mu.m from the recording medium. Since the magnetoresistance effect element is rather long (about 10 m), the resistance is proportionally high, inevitably generating a large thermal noise generally known as "Johnson noise."
The sixth object of the present invention is as follows.
Some of the conventional magnetoresistance effect elements having a magnetization-locking film made of anti-ferromagnetic material such as FeMn generate much heat when a current is supplied to them in a high density of 10.sup.6 A/cm.sup.2 or more, to exhibit but a very smally resistance changing rate.
The seventh object of the present invention is as follows.
As is known in the art, a magnetoresistance effect element utilizing spin-dependent scattering has a maximum resistance changing rate if the nonmagnetic portion sandwiched between the magnetic portions is made of Cu. As is disclosed in J. Appl. Phys. 69(8), Apr. 15, 1991, a magnetoresistance effect element of type has a maximum resistance changing rate if the magnetic surface portions are made of Co. In ordinary thin-film heads, NiFe is used as recording material and shield material NiFe has highly corrosion resistant. Hence, a shield made of NiFe was used and arranged close to a magnetoresistance effect element made of Cu and Co. Then, the Cu and Co forming the magnetoresistance effect element were electrolytically corroded during the processing of the head. Furthermore, while a data-reproducing head having only the magnetoresistance effect element was being processed, the Cu and Co forming the magnetoresistance effect element were electrolytically corroded.
The eighth object of the present invention is as follows.
When a hard disk drive (HDD) is operated at a write frequency of 10 MHz or more, the phase difference between the recording current and the recording magnetic flux increases and become problematical. This is because the pole takes a closure domain structure so that the head may record data not only by the magnetization rotation, but also by moving the magnetic barrier. No method of preventing the phase difference has been proposed.
The ninth object of the present invention is as follows. In a system that a current flows in a direction perpendicular to a film surface of a magnetoresistance effect element, electrical resistance is extremely small. Therefore, the system is impractical system.
The tenth object of the present invention is as follows.
A so-called "granular magnetic alloy," in which magnetic regions (formed mainly of Co, Ni, Fe or the like) are isolated form one anther, and nonmagnetic regions (formed mainly of Ag, Au, Cu or the like), is reported to exhibit as great a magnetoresistance effect as does a magnetic/nonmagnetic multi-layered film, more precisely a resistance changing rate of several tens per cent. This alloy cannot be practically used in a magnetic sensor (i.e., a saturation field), however, since it requires an intense magnetic field (10 kA/m or more) to have such a high resistance changing rate. An attempt has been made, wherein a bias film is laid on one surface of a granular magnetic alloy film so that the alloy film may perform its function in a low-intensity magnetic field. Contrary to the objective, the film yet requires a considerably intense saturation field to exhibit a sufficiently high resistance changing rate. Thus far no granular magnetic alloy film has been formed which can exhibit an adequate resistance changing rate in a weak magnetic field as does a spin-valve type multi-layered magnetoresistance effect film.